Electrochemical devices like supercapacitors and fuel cells are foreseen by some as necessary to the commercial realization of low-emission vehicles as well as a number of stationary power needs. Supercapacitors are useful for storing and releasing large bursts of energy, while fuel cells cleanly and efficiently convert suitable fuels to electrical energy. The unique advantages of each type of device make them, alone and together, promising for many power applications. In all cases, a balance must be struck between weight and performance, and it would be desirable to adjust manufacturing procedures of current construction materials to assure that both concerns are effectively addressed to provide a net improvement in the operation and/or economy of these devices.
The ability to design and produce key elements with consistent surface topography, density, electrical, thermal and strength characteristics depends on the provision of procedures and materials that enable close tolerance control. Tolerance control is of critical importance because tolerance variations can accumulate. It is desirable to establish procedures for treating materials that enable achieving target properties. Where this cannot be done, it is frequently necessary to compensate for out of tolerance regions during the design phase—leading to a less-than-ideal compromise on key structural and performance criteria. Often when a choice is made to add additional material in order to assure that minimum structural specifications are met, adds to the cost of the article being produced and decreases its effectiveness both per unit cost and per unit weight. In each instance where material distribution is altered to achieve improved thermal or electrical characteristics, other characteristics are affected and compromised. It would be desirable to enable more nearly meeting competing design criteria with close tolerance control.
Double-layer capacitors, sometimes also called ultracapacitors and supercapacitors, are capable of rapidly charging to store significant amounts of energy and then delivering the stored energy in bursts on demand. To be useful, they must, among other properties, have low internal resistance, store large amounts of charge and be physically strong per unit weight. There are, therefore, a large number of design parameters that must be considered in their construction. It would be desirable to enable procedures for producing starting materials for component parts that would address these concerns such that the final supercapacitor assembly could be more effective on a weight and/or cost basis.
Capacitors of the double-layer type generally include two porous electrodes, kept from electrical contact by a porous separator. Both the separator and the electrodes are immersed within an electrolyte solution. The electrolyte is free to flow through the separator, which is designed to prevent electrical contact between the electrodes and shorting of the cell. Current collecting plates are in contact with the backs of active electrodes. Electrostatic energy is stored in polarized liquid layers, which form when a potential is applied across the electrodes. A double layer of positive and negative charges is formed at the electrode-electrolyte interface.
The use of graphite electrodes in electrochemical capacitors with high power and energy density provides a number of advantages, but economics and operating efficiency are in need of improvement. Fabrication of double layer capacitors with carbon electrodes is known. See, for example, U.S. Pat. No. 6,094,788, to Farahmandhi, et al., U.S. Pat. No. 5,859,761, to Aoki, et al., U.S. Pat. No. 2,800,616, to Becker, and U.S. Pat. No. 3,648,126, to Boos, et al. The art has been utilizing graphite electrodes—but not flexible graphite sheets—for capacitors of this type for some time and is still facing challenges in terms of material selection and processing.
A continuing problem in many carbon electrode capacitors, including double-layer capacitors, is that the performance of the capacitor is limited because of the internal resistance of the carbon electrodes. While the use of carbon in the form of flexible graphite sheet has several advantages, it is desired to further reduce cell internal resistance. Internal resistance is influenced by several factors, including the high contact resistance of the internal carbon-carbon contacts, the contact resistance of the electrodes with a current collector, the surface and internal pore structure of the carbon and the material thickness. Because high resistance translates to large energy losses in the capacitor during charging and discharge, and these losses further adversely affect the characteristic RC (resistance×capacitance) time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time, it would be desirable to provide construction materials and methods that would facilitate reductions in the internal resistance.
Material selection and processing problems are also prevalent in the field of fuel cells, where construction of flow field plates (FFP's) and gas diffusion layers (GDL's) have been made of flexible graphite foil has been suggested due to an overall favorable combination of physical and electrical properties. Among the fuel cells utilizing this type of construction are ion exchange membrane fuel cells. Of these, proton exchange membrane (PEM) fuel cells are of particular interest. Cells of this type produce electricity through the chemical reaction of hydrogen with oxygen from the air. Within the fuel cell, electrodes denoted as anode and cathode, surround an ionically-conducting polymer (performing the function of an electrolyte) to form what is generally referred to as a membrane electrode assembly (or MEA). In some cells, the electrode component will also function as a GDL. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell is advantageously formed of a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes.
In operation of one of these PEM cells, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which join with the protons migrating through the proton exchange membrane and the electrons through the circuit. The result is the generation of current and the formation of water. Since the membrane is an electrical insulator, the electrons cannot directly cross the membrane, but seek the least resistance and travel through an external circuit which utilizes the electricity before the electrons join the protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen can be removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
One factor limiting the use of flexible graphite materials as components for PEM fuel cells is the definition of a pattern embossed on the flow field plates, which, if not desirably precise and regular, can create anomalies in fuel cell operation, by either permitting leaking of fluids, or not permitting sufficient fluid flow through the fuel cell. Aggravating this problem are several opposing problems. Among these, are the needs for overall structural integrity and for the surface opposed to the embossed surface to be relatively dense to reduce permeability. Thus, there is a need for a suitable structural material, which can readily be shaped at one surface to conform to the surface of an intricately-shaped mold and yet have another surface that is sufficiently dense as to be impermeable under the conditions of operation to yield an overall structure having desired characteristics in terms of electrical and thermal conductivity and the like.
To better understand the complexity of the above considerations, we present a brief description of graphite and the manner in which it is typically processed to form flexible sheet materials. Graphite, on a microscopic scale, is made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially-flat, parallel, equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly-ordered graphite materials consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites, by definition, possess anisotropic structures and thus exhibit or possess many characteristics that are highly directional, e.g. thermal and electrical conductivity and fluid diffusion. Sometimes this anisotropy is an advantage and at others it can lead to process or product limitations.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be chemically treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been chemically or thermally expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension, can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation. There is a need for processing that more fully takes advantage of these properties.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance will, once compressed, maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression.
Lower densities are often thought to be advantageous where surface detail requires embossing or molding. Lower densities aid in achieving good detail. However, strength, thermal conductivity and electrical conductivity are generally favored by more dense sheets. Typically, the density of the sheet material will be within the range of from about 0.04 g/cc to about 1.4 g/cc. It would be desirable to have a process that would enable varying densities as needed. Current technology does not lend itself easily to meet these challenges.
Flexible graphite sheet material made as described above typically exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll-pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude typically, for the “c” and “a” directions.
This considerable difference in properties, i.e. anisotropy, is directionally dependent, and is in need of control for optimizing properties for many applications. In applications such as electrodes for fuel cells, it would be of advantage if the electrical resistance transverse to the major surfaces of the flexible graphite sheet (“c” direction) were decreased, and this might be achieved by utilizing sheets of higher density. However, high density sheets might inhibit effective embossing. And, the embossing (or other shaping) operation could cause further, undesirable variations in properties. It would be desirable to enable a process which provided for both.
Thermal properties, e.g., thermal conductivity, of a flexible graphite sheet in a direction parallel to the major surfaces of the flexible graphite sheet is relatively high, while it is relatively low in the “c” direction, transverse to the major surfaces. Again, it would be desirable to alter this property in a manner consistent with effective embossing.
Flexible graphite sheet can be provided with channels, which are preferably smooth-sided, and which pass between the parallel, opposed surfaces of the flexible graphite sheet and are separated by walls of compressed expanded graphite. When the flexible graphite sheet functions as an electrode in an electrochemical fuel cell, it is placed so as to abut the ion exchange membrane so that the “tops” of the walls of the flexible graphite sheet abut the ion exchange membrane. Thus, there are products requiring dense flat surfaces and embossed surfaces, product requirements presenting different demands on a flexible graphite sheet starting material having predictable properties that can be optimized for many articles of uniform construction but must be compromised in others.
There remains a need in the art for a material which can be used in preparing flexible graphite articles, particularly those that are embossed with particular patterns thereon and, especially, to methods and materials enabling the preparation of shaped elements having predetermined density gradation and/or detail necessary for their intended functions, thereby facilitating quality control while also, preferably improving performance per unit weight for the final articles produced. If available, such needed methods and materials would aid the formation of an array of final products, some of which are useful as components in electrochemical supercapcitors and fuel cells.